This invention relates in general to a vibration motor and, in particular, to a DC brushless vibration motor having a rotor with a built-in eccentric weight.
Mechanical vibrations are required for many different applications. Vibrations for material pulverization and selection in industrial use, vibration for home massage machines, and silent notification of incoming calls and messages for mobile phone handsets and pagers, are but a few examples of mechanical vibration applications.
There are various methods that can be used to produce mechanical vibrations. One method involves the use of electric motors. For motor-based vibration generation, at least two types of vibration-producing devices are known. The first type involves the attachment of an eccentric weight to the output shaft of a conventional motor, which normally operates by rotating smoothly without any intention of vibration. Vibration is produced due to the eccentricity of the rotating part of the system as a result of the presence of the eccentric weight attached to the output shaft of the motor. However, since the eccentric weight is external to the motor housing, additional shielding means must sometimes be provided to avoid undesired impact by the rotating part of the system on the surroundings. The other type of motor-based vibration generation employs a rotor that is itself eccentric-weighted. The rotation of its rotor automatically generates vibration without any need for additional device to be attached to the rotor shaft of the motor.
Thus, a vibration motor is a type of vibration-generating device that provides mechanical vibration by the rotation of its rotor. Normally, it is an objective of a traditional motor to reduce possible vibrations. In contrast, a vibration motor has an objective of intentionally producing mechanical vibration. In the following descriptive text of this specification, the term xe2x80x9cvibration motorxe2x80x9d is used specifically to refer to the motor-based vibration devices having built-in eccentric rotors.
A DC commutator motor having a non-symmetric rotor with center of mass offset from the rotor axis of rotation is known in U.S. Pat. No. 6,169,348 to Wan (xe2x80x9cFlat Type Two-Phase Vibration Motorxe2x80x9d). Wan discloses a flat type two-phase DC commutator motor that employs a rotor with a mechanically asymmetric construction. Since the center of mass of the asymmetric rotor is not on the center axis of rotation of the rotor, mechanical vibration was produced as the rotor of the vibration motor was driven. However, Wan""s vibration motor requires the use of a commutator-brush pair so as to supply electric power to the armature coil of its rotor.
Commutation of electric power required for the operation of Wan""s vibration motor relies on the mechanical friction between the brushes and the commutator. Mechanical friction inevitably wears out both the brushes and the commutating segments of the commutator. Electrical shorting between consecutive segments of the commutator is also inevitable due to carbon accumulation resulting from mechanical friction. Moreover, intermittent electrical conduction between the brushes and the commutator segments produces undesirable electromagnetic emission that may interfere with nearby electronic devices. Without proper shielding, circuits such as the control electronics of the motor itself may fail due to excessive EM interference. Further, the wearing out of the brushes and the commutator reduces the useful lifetime of this type of vibration motor. Still further, the two phase configuration of Wan""s vibration motor requires the use of a more complex power supply circuitry that must provide two electric power supplies with phase separation. When compared to a single-phase power source, the overall cost and circuit complexity of this type of commutator-based vibration motor is relatively high.
It is therefore an object of the present invention to provide a DC brushless vibration motor for producing mechanical vibration via rotation of an eccentric rotor that avoids the mechanical wear of components thereby providing an extended service life.
It is another object of the present invention to provide a DC brushless vibration motor for producing mechanical vibration via rotation of an eccentric rotor that produces little electromagnetic interference to the surrounding environment.
It is still another object of the present invention to provide a DC brushless vibration motor for producing mechanical vibration via rotation of an eccentric rotor that operates on a simple single-phase power supply, thereby providing reduced complexity of the motor drive circuitry.
It is yet another object of the present invention to provide a DC brushless vibration motor for producing mechanical vibration via rotation of an eccentric rotor that has a low manufacturing cost.
The invention achieves the above-identified objects by providing a DC brushless vibration motor that comprises a stator, a rotor, a drive circuit and a casing. The stator has an upper magnetic pole piece, a lower magnetic pole piece and a stator field winding. The stator field winding is contained inside a ring-shaped space formed by the upper and lower magnetic pole pieces. The internal rotor has a center of mass offset from the center axis of rotation and comprises a ring magnet and an eccentric weight. The eccentric weight has a structural configuration of a ring body, and the ring magnet is securely fixed to the external peripheral surface of the eccentric weight. The drive circuit has a drive integrated circuit for controlling the rotation of the rotor by receiving externally-supplied electric power. The casing securely encloses the stator and rotor while maintaining an air gap therebetween for generating mechanical vibration when the rotor is propelled to rotate by the stator.